1. Field of the Invention
The invention relates to a method and apparatus for electrostatically charging a dielectric layer to a predetermined potential with the aid of an alternating electric field and a constant electrostatic field.
2. Description of the Prior Art
A prior art electro-photographic development process disclosed in reference "Tappi/February 1967, Vol. 50, No. 2, pages 77A-79A" teaches providing an electro-photographic layer with an electrostatic surface charge by means of an electrode to which both a high-frequency high voltage and a direct current voltage are applied simultaneously. The electrode, for example a very thin corona wire or fine metallic points, is arranged close to an insulated metallic surface and due to the alternating voltage in the air generates ions of both polarities. Those with the appropriate polarity are accelerated by the direct voltage towards the electro-photographic layer.
With a negative voltage at the electrode, strong inhomogeneities of the ions will occur close to the surface of the wire, causing fluctuations in the charge which will adversely affect the image generation on the electro-photographic layer such that, for example, a solid area in an original copy will be reproduced unevenly. By overlaying the direct current voltage field with the alternating current voltage field, the discharge voltage of the electrode is affected. Because the pre-existing direct voltage has the amplitudes of the alternating voltage superimposed on it, voltage peaks are produced which lead to a breakdown of the layer to be charged up.
From German Offenlegungsschrift No. 2,231,530 a method is disclosed for the electro-photographic recording of images on an insulating recording base which is pulled over a support electrode while above the contact point with the support electrode. The charge image is recorded by tracing electrodes on the other side of the recording base. To accomplish this, an electrode arrangement is used in which a portion of the current of the corona discharge of a discharge electrode reaches the recording base through the opening of a slit aperture formed by the electrodes, and there produces a charge above the line of contact between the recording base with the support edge. The partial discharge current is controlled by electric image signals via the aperture formed by four flat electrodes. For this at least one of the electrodes is subdivided into a number of conductor strips via which the signal voltage is supplied.
German Offenlegungsschrift No. 2,423,245 describes a method for the electro-photographic recording of images on an insulating recording base by means of a corona discharge from which a part of the discharge current is removed via a slit aperture and used for charging the recording base. Here, too, the image-dependent charging is done by control voltages at a tracing electrode which is located on the side of the recording base facing away from the slit and which is in contact with the recording base. The electric contact between the tracing electrode and the insulating recording base is facilitated by supplying a conductive contact fluid to the contact point. In this arrangement, the charging of the recording base can take place in streaming nitrogen.
It is the object of the invention to produce a method for the gentle and safe electrostatic charging of insulating dielectric layers, while avoiding breakdowns, wherein the magnitude of the charging current and the charge distribution can be reproduced in a changeable and highly accurate manner with pronounced linearity between the charging current and the pre-existing direct current voltage.